Muslims are converging on Mecca for the hajj, Islam's annual pilgrimage that has been plagued by fires, stampedes and occasional riots. The risks are even greater this year, with Saudi authorities also worried about diseases and terrorism.

More than 2 million Muslims are expected for the pilgrimage that will climax late this month, a devotion required once in the lifetime of every able-bodied Muslim who can afford it.

In a ritual lasting four to six days, pilgrims circle the Kaaba (search), the big stone structure that Muslims face during their five daily prayers; pray at Mount Arafat; throw pebbles at three pillars symbolizing the devil; and slaughter a camel, sheep or cow to mark the beginning of Eid al-Adha (search), the Feast of the Sacrifice.

The huge numbers of pilgrims in recent decades have produced hazards. A stampede in 1990 killed 1,426 people, and high winds in 1997 swept fire through a sprawling tent city for pilgrims, killing more than 340 and injuring 1,500.

The new challenges this year include infectious diseases as diverse as Ebola and SARS, and officials will keep close watch for any illness outbreaks among pilgrims.

Saudi Arabia has set up a mammoth tent city in Mina, with more than 40,000 white, fireproof tents to house the pilgrims. Dozens of trucks will roam the streets, offering food and drink.

Mohammed Abdulaziz, head of a hajj (search) group for American pilgrims in the Saudi city of Jiddah, said that despite the risks, more American pilgrims are arriving this year than ever before.

"Muslim Americans have not shied away despite the recent attacks in the kingdom," he said. "Their numbers have risen slightly from last year."

Saudi Arabia has been struck by two terrorist bombings in the past year that killed 51 people, including eight Americans, at housing compounds for foreigners. Saudi and U.S. officials have blamed the Al Qaeda network of Saudi exile Usama bin Laden.

An Interior Ministry official said last week that authorities had discovered Al Qaeda camps in the Saudi desert that trained militants to carry out terror attacks. Saudi Arabia has arrested hundreds of suspects since suicide bombings in May.

In a statement Monday, Saudi Arabia's King Fahd urged pilgrims not to cause trouble, warning them to follow the kingdom's laws and "avoid actions that would desecrate the pilgrimage."

Saudi authorities are concerned about possible anti-American riots because of the U.S.-led wars in Afghanistan and Iraq following the Sept. 11 terrorist attacks. Many Muslims consider the U.S. wars a campaign against their faith.

"The hajj has historically been a period of potential trouble and instability, and my sense is that Saudi authorities are taking the security challenges before them very seriously," said Daniel Benjamin, a terrorism expert at the Center for Strategic and International Studies (search) in Washington who recently visited Saudi Arabia.

In 1987, 402 people, mostly Iranian pilgrims, were killed and 649 wounded when Saudi security forces clashed with Iranians staging anti-U.S. demonstrations. The Saudis later broke off relations with Iran, but the two nations restored ties after reformist Mohammad Khatami became Iran's president in 1997.

Saudi Health Minister Hamad al-Manie told reporters last week that pilgrims from the Republic of Congo will not be allowed at the hajj this year because of a recent Ebola epidemic there.

But he said there was no ban on Chinese pilgrims, despite the re-emergence of severe acute respiratory syndrome. SARS killed 774 people worldwide last year, and three new cases have emerged in China since December.

Scanners to detect arriving pilgrims with high body temperatures — a possible sign of SARS — have been installed at airports, where 500 health workers have been deployed, said Khaled bin Saleh al-Sawaf, director of health services in Jiddah.

With the ouster of Saddam Hussein's regime in Iraq, some 30,000 Iraqis are expected to make the pilgrimage this year, the largest contingent in 35 years.